The fine structure of 1s22snl states with high angular momentum l has been measured for the first time in a high-Z four-electron system by studying the n=6-7 transition in the S12+ ion. Strong perturbations by 1s22pnl states are observed and are interpreted using results of new multiconfiguration Dirac-Fock calculations. Implications for the spectroscopy of highly ionized atoms and the determination of high-Z atomic core polarizabilities are discussed. © 1985 The American Physical Society.